The invention relates to a light metal cylinder block having at least one wear-resistant and tribologically optimised cylinder running face, comprising a light metal matrix alloy and a powder material which contains a hardening material and which is present on the light metal matrix in the form of a finely dispersed surface layer containing primary silicon precipitations. The invention also relates to a method by which to produce the blocks and a device with which to produce the blocks.
According to EP 0 837 152 A1 (Bayerische Motoren Werke AG), there is known a method of coating a component of an internal combustion engine, which component consists of an aluminium alloy. A laser beam is directed in such a way that it does not directly reach the surface of the component to be coated, but first hits a powder beam. As a result of the energy of the powder beam, the powder is transformed completely from the solid phase into the liquid phase, so that the powder, when hitting the component surface, is separated in the form of fine droplets as a coating material on the component surface, which fine droplets, as a result of the solidification conditions, solidify so as to be partially amorphous.
Therefore, in the case of the prior art method, the powder is not alloyed into the surface layer of the component, but there takes place a phase transformation of the coating material on its way to the surface, with the aluminium silicon powder being liquefied in the laser beam. When the powder solidifies on the surface, the object is to release a finely dispersed silicon, a so-called primary silicon.
Depending on the cooling speed, the purpose is to produce silicon crystals whose size ranges between 1 to 5 xcexcm. However, rapid cooling, as required, cannot be achieved in practice because of the energy of the laser beam acting on the component to be coated. In consequence, the substrate surface heats up very quickly and therefore cannot discharge the heat of the arriving Si melt quickly enough, so that instead of a crystalline phase and primary crystals, there occurs an amorphous phase.
In accordance with the embodiment of the BMW patent, in the case of an applied layer thickness of 3 mm, approximately 50% are removed to achieve a smooth, planar surface of the coating material (column 6, lines 10 to 15). This means high removal losses and an unused boundary zone as a result of the pronounced waviness of the material applied drop-wise, which constitutes an additional disadvantage.
Furthermore, it is known from EP-A-0 221 276 to render an aluminium alloy more wear-resistant by remelting its surface layer by laser energy. A layer consisting of a bonding agent, silicon in powder form, copper and titanium carbide is applied to the surface and subsequently melted into the surface by laser. According to the embodiments listed, TIC is added in amounts ranging between 5% and 30% and achieves a considerable increase in the surface hardness. However, from a tribological point of view, the extremely high cooling speed during laser remelting achieves a high degree of core fineness, but a sufficient amount of primary silicon cannot be produced with this method. Therefore, laser remelting is not suitable for producing cylinder running faces of reciprocating piston engines consisting of AlSi alloys with supporting plateaus of primary silicon and set-back regions containing lubricants.
EP 0 411 322 describes a method for producing wear-resistant surfaces of components made of an AlSi alloy, which method is based on the previously mentioned EP 0 211 276, but prior to carrying out the laser remelting process, the layer is provided with an inoculation agent (germ forming agent) for primary silicon crystals. The following substances are mentioned as inoculation agents or germ forming agents: silicon carbide, titanium carbide, titannitride, boron carbide and titanium boride.
In a preferred embodiment, the coating is produced by silk-screen technology in the form of a peel-off coating and applied to the surface of the component concerned. The coating thickness can preferably amount to 200 xcexcm and the melting-in depth can amount to 400 to 600 xcexcm. Use is made of a linearly focussed laser beam in an inert atmosphere to be able to achieve a melting-in depth of 400 xcexcm. In the example given, the silicon content in the alloyed zone amounted to 25% with a nickel content of 8% (hardness in excess of 250 HV).
As already mentioned above, it is necessary, in the case of the latter processes of remelting and melting-in, to carry out a cooling process while applying a coating on to the matrix alloy in order to achieve the required finely dispersed segregations of primary silicon. Because of the addition of inoculation agents, reactions can take place on the aluminium surface. In addition, the coating measures cannot always be applied to curved surfaces.
EP 0 622 476 A1 proposes a metal substrate with a laser-induced MMC coating. The MMC coating comprises a coating thickness between 200 xcexcm and 3 mm and contains homogeneously distributed SIC particles; in a preferred embodiment, up to 40% by weight of SiC is contained in the MMC coating in the form of homogeneously distributed SIC particles. For production purposes, the powder mixture containing SiC powder and pre-alloyed AlSi powder is heated in a laser beam, with the heat content required for producing a homogeneous alloy from the powder mixture being provided by the powder applied to the substrate. Products containing hard metal materials such as SiC comprise a very high hardness which is disadvantageous for the wear behaviour of the piston rings. Furthermore, machining is very complicated and expensive because the top layer of the ceramic particles has to be removed in order to achieve a functionable, splinter-free running face.
The invention includes a light metal cylinder block having at least one wear-resistant and tribologically optimized cylinder running face, comprising a light metal matrix alloy with a finely dispersed surface layer containing primary silicon phases, wherein the primary silicon comprises uniformly distributed approximately roundly formed grains with a medium grain diameter ranging between 1 and 10 xcexcm and wherein the surface layer contains about 10 to about 14% AlSi eutectic, about 5 to about 20% primary silicon, the remainder being pure Al phase, and wherein the minimum hardness of the surface amounts to about 160 HV.
Furthermore, the invention includes a method of producing a light metal cylinder block having at least one wear-resistant and tribologically optimized cylinder running face, comprising a light metal matrix alloy and a powder material which contains a hard material and which is present in the form of a finely dispersed surface layer with primary silicon precipitations in the light metal matrix, using a gravity, low-pressure or high-pressure die casting method with subsequent surface treatment by parallel laser and powder beams wherein the laser beam is guided in a strip width of at least 2 mm transversely to the direction of feed across the matrix surface and wherein it is only in the point of impact of the laser beam on the light metal matrix surface in a contact time of 0.1 to 0.5 seconds, that the powder is heated to melting temperature and alloyed-in.
The invention also includes a device for coating the a running surface of a hollow cylinder, comprising powder supply means (1), a laser beam device (2) and a focusing system (3) with a deflecting mirror (4), characterized in that the powder supply means (1) and the laser beam device (2) are guided parallel relative to one another in the radial and axial direction of the hollow cylinder; that the focusing system (3) comprises a linear beam exit with a beam width of 2.0 to 2.5 mm; and that the powder supply means are provided with a metering device by means of which the volume flow of the powder can be set as a function of the speed of feed of the laser beam.